Spark plug insulators are typically made from hard dielectric materials, such as ceramic materials made from alumina, and are designed to provide mechanical support for a center electrode while also providing electrical isolation between the center electrode and a metallic shell. The dielectric strength or dielectric breakdown strength of a spark plug insulator generally refers to the applied electrical field at which the insulator breaks down and experiences a rapid reduction in electrical resistance. Because spark plug insulators are expected to electrically isolate the center electrode from the metallic shell, the dielectric strength of the insulator is an important characteristic of the component and can affect the overall performance of the spark plug.
The dielectric strength of an insulator can be affected by pores, relics and/or other defects in the ceramic microstructure of the component. Dry pressing is a conventional method for manufacturing spark plug insulators, however, this method is somewhat prone to the formation of pores. Other manufacturing methods, such as extruding, have shown some signs of reducing the number of pores in the ceramic microstructure, but these methods have traditionally been unable to produce an insulator structure that includes certain features like a stepped internal bore within the insulator. A stepped internal bore is needed to properly seat and secure the center electrode within the insulator.